STRUCTURAL DESIGN CRITERIA
5.5 SOIL-STRUCTURE INTERACTION EFFECTS
5.5.1 General
5.5.3.3 Design Values: The design values of the modified shears, moments, deflections, and story drifts should be determined as for structures without interaction by taking the square root of the sum of
the squares of the respective modal contributions. In the design of the foundation, the overturning moment at the foundation-soil interface determined in this manner may be reduced by 10 percent as for structures without interaction.
The effects of torsion about a vertical axis should be evaluated in accordance with the requirements of Sec. 5.3.5 and the P-delta effects should be evaluated in accordance with the requirements of Sec.
5.3.7.2, using the story shears and drifts determined in Sec. 5.5.3.5.
Other Methods of Considering the Effects of Soil Structure Interaction: The procedures proposed in the preceding sections for incorporating the effects of soil-structure interaction provide sufficient flexibility and accuracy for practical applications. Only for unusual structures of major importance, and only when the requirements indicate that the interaction effects are of definite
consequence in design, would the use of more elaborate procedures be justified. Some of the possible refinements, listed in order of more or less increasing complexity, are:
1. Improve the estimates of the static stiffnesses of the foundation, Ky and K2, and of the foundation damping factor, $o, by considering in a more precise manner the foundation type involved, the effects of foundation embedment, variations of soil properties with depth, and hysteretic action in the soil. Solutions may be obtained in some cases with analytical or semi-analytical formulations and in others by application of finite difference or finite element techniques (Blaney et al., 1974;
Luco, 1974; Novak, 1974; Veletsos and Verbic, 1973). It should be noted, however, that these solutions involve approximations of their own that may offset, at least in part, the apparent increase in accuracy.
2. Improve the estimates of the average properties of the foundation soils for the stipulated design ground motion. This would require both laboratory tests on undisturbed samples from the site and studies of wave propagation for the site. The laboratory tests are needed to establish the actual
variations with shearing strain amplitude of the shear modulus and damping capacity of the soil, whereas the wave propagation studies are needed to establish realistic values for the predominant soil strains induced by the design ground motion.
3. Incorporate the effects of interaction for the higher modes of vibration of the structure, either approximately by application of the procedures recommended in Bielak (1976), Roesset et al. (1973), and Tsai (1974) or by more precise analyses of the structure-soil system. The latter analyses may be implemented either in the time domain by application of the impulse response functions presented in Veletsos and Verbic (1974). However, the frequency domain analysis is limited to systems that respond within the elastic range while the approach involving the use of the impulse response functions is limited, at present, to soil deposits that can adequately be represented as a uniform elastic halfspace. The effects of yielding in the structure and/or supporting medium can be considered only approximately in this approach by representing the supporting medium by a series of springs and dashpots whose properties are independent of the frequency of the motion and by integrating numerically the governing equations of motion (Parmelee et al., 1969).
4. Analyze the structure-soil system by finite element method (Seed et al., 1974 and 1977; Vaish and Chopra, 1974), taking due account of the nonlinear effects in both the structure and the supporting medium.
It should be emphasized that, while these more elaborate procedures may be appropriate in special cases for design verification, they involve their own approximations and do not eliminate the uncer- tainties that are inherent in the modeling of the structure-foundation-soil system and in the specification of the design ground motion and of the properties of the structure and soil.
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